1. Field of the Invention
The present invention relates to a lithium secondary battery comprising an electrode assembly which consists of a cathode, an anode and a separator laminated therebetween, and a battery case in which the electrode assembly is placed.
2. Description of the Related Art
Generally, lithium secondary batteries have a structure comprising a sheet-shaped cathode having an active material-contained layer which is formed on a current collector and which uses a composite oxide of lithium and a transition metal as the active material, a sheet-shaped anode having an active material-contained layer which is formed on a current collector and which uses a carbon material as the active material and a separator having lithium ion conductivity and electron insulation therebetween, and these are placed in a metallic battery case and sealed in such a manner that a cathode and an anode are electrically disconnected from each other, so that the lithium secondary batteries are characterized by their high capacity, high voltage, high output, and the like.
The lithium secondary battery having such a high energy density generally have various safety mechanisms in order to secure their safety under abnormal conditions. For example, these mechanisms include: a safety valve mechanism according to which when the pressure inside the battery case rises extraordinarily, the safety valve transforms in order to mitigate the internal pressure or to safely release the internal gas outside (Japanese Patent Kokai Publication 2-288063); a pressure-sensitive cutoff mechanism according to which when the pressure inside the battery case rises extraordinarily due to an abnormal charging/discharging, the internal battery circuit is cut off in order to stop the abnormal charging/discharging (Japanese Patent Kokai Publication 2-288063); and a temperature-sensitive cutoff mechanism according to which when the battery temperature rises extraordinarily due to an abnormal charging/discharging, the resistance of the internal battery circuit is raised in order to stop the abnormal charging/discharging (Japanese Patent Kokai Publication 5-74493).
When these cutoff mechanisms of the internal battery circuit based on the pressure or temperature operate in response, for example, to the occurrence of trouble under a full-charged condition, the electrode assembly inside the battery is left under a full-charged condition even though it does not show a voltage at the external battery terminal because the internal battery circuit is cut off. Accordingly, if the battery is pierced with a conductive material such as a nail, crushed by an external pressure, or heated from the outside, there is a problem that the pressure may increase extraordinarily.
On the other hand, a battery where the terminals of the cathode and the anode are electrically connected via a diaphragm valve which transforms in accordance with an increase in the battery's internal pressure has been suggested (Japanese Patent Kokai Publication 7-201372). However, after the above-mentioned safety valve mechanism operates, the pressure inside the battery case decreases and this pressure-sensitive short-circuit mechanism may fail to make the short-circuit mechanism operate.